1. Field of the Invention
This invention concerns an enhanced oil recovery process and more specifically a surfactant flooding enhanced oil recovery process. Still more specifically, this invention is concerned with an oil recovery process suitable for use in subterranean formations whose water is naturally fresh, or in formations in which the water salinity can be reduced to a relatively fresh level by preflush treatment, permitting the use of an aqueous fluid containing petroleum sulfonate, the petroleum sulfonate having a preferred average equivalent weight value.
2. Description of the Prior Art
It is well known in the prior art that when a subterranean petroleum formation containing sufficient petroleum and having adequate permeability to permit commercial exploitation thereof is discovered, and primary petroleum recovery is accomplished to the normal commerical termination point and waterflooding is thereafter applied to the formation, only from 30 to 70 percent of the oil normally present in the formation is recovered and significant quantities of petroleum remain in the formation thereafter. Many prior art references teach the use of an enhanced recovery or tertiary recovery program in which an aqueous fluid, which may be a pure solution, or a water external emulsion or an oil external emulsion is injected into the formation to displace and recover additional petroleum from the formation over the obtainable by primary recovery and by waterflooding. Although references are found in the prior art to the use of many surfactants, petroleum sulfonates is still the preferred surfactant for enhanced oil recovery at the present time because of the relatively low cost of petroleum sulfonate as compared to synthetic surfactants. For example, U.S. Pat. No. 3,302,713 teaches the use of a particular petroleum sulfonate prepared from a specific boiling range fraction of petroleum feedstock. It is also taught in the prior art that optimum recovery and surfactant flooding processes using petroleum sulfonate is achieved if the petroleum sulfonate comprises a mixture of at least one petroleum sulfonate which is predominantly water soluble and at least one petroleum sulfonate which is predominantly oil soluble. U.S. Pat. No. 3,434,542 teaches the use of petroleum sulfonate whose median equivalent weight is from 375 to 430, and contains components having equivalent weights in the range of from 290 to 590 with no more than 10% being less than 290 and no more than 15 percent being greater than 590. The surfactant fluid also contains polysaccharides as a viscosifier for mobility control. U.S. Pat. No. 3,468,377 describes an oil recovery process employing about the same petroleum sulfonate in an aqueous fluid which also contains sodium chloride, sodium carbonate and inorganic phosphates. The prior art recognizes generally that the average equivalent weight of the particular petroleum sulfonate utilized influences the ultimate oil recovery efficiency, and implicitly or explicitly relates the oil recovery displacement efficiency to the capability of the particular petroleum sulfonate material utilized in reducing the interfacial tension between the formation petroleum to be recovered and the aqueous fluid, e.g., the formation brine present in the formation at the time the surfactant solution contacts the formation petroleum.
Despite encouraging laboratory results, field use of surfactant flooding techniques has never been entirely satisfactory from a commerical point of view, principally because the value of the additional oil recovered is insufficient to justify the high cost of the surfactant materials injected into the formation.
In view of the present inadequacy of gross domestic productive capability to satisfy our needs for petroleum, it is of paramount importance that surfactant flooding technology be developed which will permit commercial production of the oil remaining in a subterranean petroleum formation after conclusion of conventional primary and secondary recovery operations.